The Corbomite Maneuver

The Enterprise encounters a strange cube, moving through space without apparent means of propulsion. When it begins emitting deadly radiation, Kirk orders it destroyed. Later, they encounter the ship from which the cube originated: the Fesarius, the flagship of the First Federation. Its commander, Balok, indicates that the Enterprise will be destroyed for behaving violently.

In the end, desperate, Kirk tries bluffing: he claims the all Federation starships contain a substance, corbomite, which will destroy any attacking ship. Balok decides instead to tow the Enterprise, but it is able to escape. In doing so, Balok's ship is–apparently–badly damaged, and when Kirk, Spock, and McCoy transport over to render aid, Balok reveals that it was all a ruse, to test them, and they can look forward to cordial relations.